1. Field of the Invention
The present invention relates to a black level stretching apparatus for image signal processing systems, and more particularly, to an apparatus and method for stretching the black level of an image signal, which are capable of eliminating flicker caused by a sudden change in image signal levels.
2. Description of the Related Art
In general, the dynamic range of an input image signal is narrower than the fixed range of the hardware for processing image signals. Here, the fixed range of the hardware means the range from the minimum level to the maximum level of an image signal, which can be displayed by an image signal processing system. When an image signal whose dynamic range is narrower than the range of the hardware is input to an image signal processing system, the dynamic range of the image signal is stretched close to the range of the hardware, to thereby increase the resolution of the image signal.
On the other hand, the image quality is more important in dark regions than bright regions of a screen, so approaches have been made to increase the resolution of the dark region. Increasing the resolution of dark region of a screen is referred to as xe2x80x9cblack stretchingxe2x80x9d or xe2x80x9cblack expandingxe2x80x9d. In addition, when almost no image signal exists in the dark region, the entire screen seems white and dull. In other words, when the dynamic range of the image signal is concentrated on the bright region, the dynamic range of the image signal in the dark (xe2x80x9cblackxe2x80x9d) region is expanded so as to improve the image quality. The term xe2x80x9cimage signalxe2x80x9d used through this specification refers to the xe2x80x9cbrightnessxe2x80x9d of the image signal, which is one of a variety of signals used to express the image.
There are two typical black level stretching techniques. A first method is to detect the minimum and maximum values in the dynamic range of an input image signal, calculate a tilt point, from which black level stretching starts, and stretch the image signal whose level is lower than the tilt point in proportion to the difference between the tilt point and the levels of the image signal. This conventional black level stretching technique is illustrated in FIG. 1. As shown in FIG. 1, a tilt point Yt, from which black level stretching starts, is calculated by formula (1):
Yt=Y min+RT*(Y maxxe2x88x92Y min) xe2x80x83xe2x80x83(1) 
where Ymin represents the minimum value of an input image signal Yin, RT represents a ratio of dark region to the entire dynamic range of the input image signal, and Ymax represents the maximum value of the input image signal Yin.
In the black level stretching method illustrated in FIG. 1, the black level of the image signal Yin is expanded using a predetermined gradient, for example, G1, G2 or G3, in accordance with the dynamic range of the input image signal Yin irrespective of the hardware range of the image processing system used, as expressed by formula (2):
Yout=Yin+G*(Yinxe2x88x92Yt)xe2x80x83xe2x80x83(2) 
where Yout represents the result of black level stretching of the input image signal Yin, and G represents a predetermined gradient, for example, G1, G2 or G3. In FIG. 1, dashed lines 12, 14 and 16 represent the results of black level stretching of the input image signal Yin using gradients G1, G2 and G3, respectively. In other words, the black level below the tilt point Yt is expanded in proportion to the difference between the black level of the input image signal Yin and the tilt point Yt.
In the case where a screen has a consistent dark tone, for example, when a night scene is displayed, i.e., the dynamic range of an input image signal is narrow, the black level of the image signal is slightly stretched. Meanwhile, when a larger amount of contrast appears in a displayed image, i.e., when the dynamic range of an input image signal is wide, the black level of the image signal is more widely stretched, thereby displaying a more distinct image. However, even when there is no need to stretch the dark region, i.e., when the dynamic range of the input image signal is as wide as the range of the hardware, the black level can be undesirably stretched. In this case, almost all signal levels of the dark region become lower than the minimum value of the hardware range, so that image detail in the dark region cannot be seen.
On the other hand, a second black level stretching technique is to stretch the minimum value Ymin of a screen to the minimum level of the hardware range, thereby stretching the dark region below the tilt point Yt. The second black level stretching technique is illustrated in FIG. 2. As shown in FIG. 2, the minimum value Ymin of a screen is always stretched to the minimum level of the fixed hardware range to stretch the black level of the image signal, as expressed by formula (3):                     Yout        =                                            Yt                              Yt                -                                  Y                  ⁢                                      xe2x80x83                                    ⁢                  min                                                      *                          (                              Yin                -                Yt                            )                                +          Yt                                    (        3        )            
where   Yt      Yt    -          Y      ⁢              xe2x80x83            ⁢      min      
is the gradient of the line 22, which is calculated using two points B and Yt. The right term of formula (2) can be rearranged in a similar way to formula (2), as expressed by formula (4):                     Yout        =                  Yin          +                                    (                                                Yt                                      Yt                    -                                          Y                      ⁢                                              xe2x80x83                                            ⁢                      min                                                                      -                1                            )                        *                          (                              Yin                -                Yt                            )                                                          (        4        )            
where       Yt          Yt      -              Y        ⁢                  xe2x80x83                ⁢        min              -  1
corresponds to the gradient G of formula (2).
In the second black level stretching technique, the minimum level of signal to which the signal is stretched cannot be lowered than the minimum level of the hardware range, i.e., zero. For this reason, unlike the first conventional black expansion technique, although an image signal having a wide dynamic range is input, the black level stretched signal does not drop beyond the minimum level of the hardware range. However, even for an input image signal having a narrow dynamic range, such as a night scene having a consistent tone, the minimum value Ymin of the image signal is also dropped to the minimum level of the hardware range, to thereby stretch the black level of the signal. As a result, the dark region of the screen becomes dull.
Both the first and second black level stretching techniques described above cause an error in extracting the minimum and maximum values of a screen due to impulse noise present in the screen, so that flicker occurs. In addition, when a continuous black or white level signal, such as caption data, is displayed in a particular place of the screen, the extraction of the minimum and maximum values of the screen is adversely affected by the displayed black or white level signal. As a result, black level stretching cannot be accurately performed.
To solve the above problems, it is a first objective of the present invention to provide an apparatus and method for adaptively stretching the black level of an input image signal according to the dynamic range of the input image signal, so that occurrence of flickering in black level stretching, which is caused due to impulse noise or caption data, can be minimized.
It is a second objective of the present invention to provide a black level stretching apparatus and method, in which the range of black stretching is varied by users and the occurrence of flickering in black level stretching, which is caused due to impulse noise or caption data, can be minimized.
According to an aspect of the first objective of the present invention, there is provided an adaptive black level stretching apparatus comprising: a minimum and maximum value extraction unit for receiving an image signal, extracting the minimum and maximum values of each field of the image signal, and outputting the extracted minimum and maximum values as field minimum and maximum values, respectively; and an adaptive black level stretch unit for (i) calculating a tilt point which delimits a dark region that is stretched using the field minimum and maximum values, (ii) calculating a first gradient for use in stretching the field minimum value to a predetermined value using the field minimum value and the tilt point, (iii) selecting the first gradient or a second gradient defined by users according to the dynamic range of the image signal, and (iv) stretching the black level of the image signal using the selected gradient.
In another embodiment, the adaptive black level stretching apparatus comprises: a first low-pass filter unit for low-pass-filtering an image signal to remove impulse noise present in lines, and extracting the line minimum and maximum values of each line from the low-pass-filtered image signal; a second low-pass-filter unit for low-pass-filtering the line minimum and maximum values, extracting the minimum and maximum values from the low-pass-filtered line minimum and maximum values, and outputting the extracted minimum and maximum values as the field minimum and maximum values, respectively; and an adaptive black level stretch unit for calculating a tilt point which delimits a dark region that is stretched using the field minimum and maximum values, and stretching the black level of the image signal if the image signal level is lower than or equal to the tilt point.
According to another aspect of the first objective of the present invention, there is provided a method for stretching the black level of an input image signal, comprising the steps of: (a) receiving the image signal and extracting the minimum and maximum values of each field of the image signal to obtain field minimum and maximum values; (b) extracting a tilt point which delimits a dark region that is stretched using the field minimum and maximum values; (c) calculating a first gradient for stretching the field minimum value to a predetermined value, using the field minimum value and the tilt point; (c) selecting the first gradient or a second gradient defined by users, according to the dynamic range of the input signal; and (e) stretching the black level of the image signal using the selected gradient.
According to another aspect of the second objective of the present invention, there is provided a method for stretching the black level of an input image signal, comprising the steps of: (a) receiving the image signal and extracting the minimum value of each field of the image signal to obtain a field minimum value; (b) receiving the image signal and extracting the average value of the image signal levels of each field that belongs to a dark region to be stretched, and outputting the extracted average value as a dark region average value; (c) extracting a tilt point which delimits the dark region that is stretched using the field minimum value and the dark region average value; (d) calculating a pre-gradient for stretching the field minimum value to a predetermined value, using the field minimum value and the tilt point; (e) calculating a gradient by multiplying the pre-gradient by a predetermined gradient grain; and (f) stretching the black level of the image signal using the gradient.
According to another aspect of the second objective of the present invention, there is provided a method for stretching the black level of an input image signal, comprising the steps of: (a) receiving the image signal and extracting the minimum value of each field of the image signal to obtain a field minimum value; (b) receiving the image signal and extracting the average value of the image signal levels of each field that belongs to a dark region to be stretched, and outputting the extracted average value as a dark region average value; (c) extracting a tilt point which delimits the dark region that is stretched using the field minimum value and the dark region average value; (d) calculating a pre-gradient for stretching the field minimum value to a predetermined value, using the field minimum value and the tilt point; (e) calculating a gradient by multiplying the pre-gradient by a predetermined gradient grain; and (f) stretching the black level of the image signal using the gradient.
The above objectives and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:
FIG. 1 illustrates a conventional black level stretching technique;
FIG. 2 illustrates another conventional black level stretching technique;
FIG. 3 is a block diagram of a preferred embodiment of an adaptive black level stretching apparatus according to the present invention;
FIG. 4 is a detailed circuit diagram of the adaptive black level stretching apparatus of FIG. 3;
FIG. 5 illustrates the characteristics of the horizontal LPF of FIG. 4 with reference to an image signal;
FIG. 6 illustrates the relationship between the gain and the cut-off frequency of the horizontal LPF 34 shown in FIG. 4;
FIG. 7 illustrates the step response characteristics of the vertical LPF of FIG. 4;
FIG. 8 illustrates the step response characteristics of the field LPF of FIG. 4;
FIG. 9 illustrates the black level stretching of an image signal by the adaptive black stretching apparatus of FIG. 4;
FIG. 10 is a block diagram of another preferred embodiment of a black level stretching apparatus according to the present invention;
FIG. 11 is a detailed circuit diagram of the black level stretching apparatus of FIG. 10;
FIG. 12 illustrates the black level stretching of an image signal by the black level stretching apparatus of FIG. 11;
FIG. 13 is a flowchart illustrating a black level stretching method used by the adaptive black level stretching apparatus of FIG. 4; and
FIG. 14 is a flowchart illustrating a black level stretching method used by the black level stretching apparatus of FIG. 11.